During the past two decades, carbon fiber-reinforced carbon-carbon composites (also referred to as carbon matrix composites) together with composites in which the matrix is ceramic material or metal ( ceramic and metal matrix composites) have been widely employed for solving difficult aerospace problems; and they show excellent potential for structural uses at high temperatures as lightweight, high performance materials.
In carbon-carbon composites, the carbon matrix is reinforced by carbon fiber incorporated therein. The carbon fibers impart directionally oriented strength, while the carbon matrix holds the fibers together and allows for improved thermal resistance. The composites exhibit high fracture toughness and wear resistance and are extremely stable at high temperatures upwards of 3000K (2727.degree. C.). High temperature applications are, however, severely limited due to their degradation by oxidizing gases. In oxidizing environments at high temperatures carbon readily chemisorbs oxygen and desorbs the carbon oxides formed. This process leads to the erosion of carbon-carbon composites. Mass losses of only several percent lead to the rapid decline of intralaminar properties (see e.g. Eitman, D. A. and W. C. Loomis, "Advanced Oxidation Protection Systems for Structural Carbon-Carbon Composites", AFWAL-7R-86-4069, p 107, September, 1986) The main reason for this is that the thermal expansion between structural carbon fibers and the composite matrix in carbon-carbon composites leads to the opening of an interface during the manufacturing process or during application, whenever the composite is subjected to elevated temperatures. In oxidizing environments, oxidation takes place primarily at such exposed interfaces.
Similar problems limit the practical use of ceramic and metal matrix composites. In oxidizing environments at high temperatures, these composites also undergo rapid degradation, primarily due to oxidation at the fiber-matrix interface.
Conventionally, an external coating is applied to the surface of the different carbon fiber-reinforced matrix systems. In case of carbon-carbon composites, typically a two-layer external coating is employed, wherein the outer layer is usually silicon carbide (SIC) or silicon nitride (Si.sub.3 N.sub.4). The inner layer is a borate glass sealant precursor, such as boron (B), boron carbide (B.sub.4 C) or silicon boride (SiB.sub.6). Upon oxidation, the outer layer is converted into a silica (SiO.sub.2) coating that prevents both the inward diffusion of oxygen and the outward diffusion of the carbon oxides. However, thermal expansion mismatches between the outer layer and the carbon-carbon substrate invariably lead to cracks in the outer layer. The function of the inner boron-rich layer is to form a borate sealant upon oxidation that seals the cracks that open in the outer layer. Although this sealant is required for satisfactory coating performance, most coating failures can be traced to chemical attack of the outer layer by the sealant, separation of the outer layer when the sealant expands on reaction with moisture or mechanical disbonding at the sealant layer-substrate interface.
Another approach is the incorporation of oxidation inhibitors, most commonly boron-rich sealant glass precursors as particulates into the matrix phase. In combination with the above-described external coatings, the substrate inhibitors represent a backup oxidation protection, since upon oxidation the sealant glass precursor particulates form a low melting borate sealant inhibitor that, ideally, should coat and protect both the carbon matrix and the fibers. This secondary oxidation protection is particularly important in the temperature range of about 400.degree. to 900.degree. C. wherein the external crack sealant is prone to failure. The usefulness of internal substrate inhibitors is, however, questionable since it is difficult to get the sealant glass precursor particulates within yarn bundles where they should form fiber protecting sealants. Moreover, the oxidation inhibiting sealant glass must have a special combination of properties to protect both the matrix and the fibers. It must cover exposed carbon in the matrix and fibers, i.e. it has to be able to wet carbon. Due to the very low surface energy of carbon for proper wetting the sealant glass must have an even lower surface energy: therefore, the choice of applicable materials is strongly limited. The only chemicals with the desired properties are borate glasses that are liable to absorb moisture. This can lead to coating failure caused by swelling of the glass or by the sudden release of steam upon heating the composite.
Since recent experimental results have indicated that the rate of fiber oxidation in carbon-carbon composites is a significant contributor to the oxidation of the composites, the oxidation protection of fibers is in the center of interest. The effect of boron doping on carbon fiber microstructure and reactivity is discussed by Jones et al., "The Effect of Boron on Carbon Fiber Microstructure and Reactivity", Journal de Chemie-Physique, January 1988. The authors found that low boron concentrations decrease the oxidation of carbon fibers considerably; however, their study did not extend to the investigation of the influence of boron dopant on mechanical and physical properties. Moreover, boron doping is carried out with special equipment, under ultra-pure conditions, and at high temperatures (2773K); hence, this technique is expensive and is not compatible with normal fiber manufacturing and finishing technology.
Another way Of providing oxidation protection is the deposition of an oxidation resistant coating on the fiber surface, e.g. by chemical vapor deposition (CVD). The CVD of a continuous coating directly upon a carbon (or ceramic) fiber surface produces thermal expansion incompatibility between the fiber and the applied refractory coating material [particularly when high strength, polyacrylonitrile (PAN) based carbon fibers are used)], which is expected to form cracks from the thermal stress produced during cool down in case of high temperature (1400.degree. to 1650.degree. C.) coatings. The use of CVD is also limited by the inherent chemical reaction stoichiometry requirements for consistent vapor deposition of the same coating. For better performance, multiple layers of different inhibitors can be produced by CVD while the deposition of mixtures of different inhibitors within a common layer is very difficult to produce. An additional disadvantage of chemical vapor deposition is its inherently high cost due to the need of high temperature vacuum reactor operating equipment. Thus, the integration of this step into a fiber manufacturing process is difficult and rather expensive.
The deposition of oxidation resistant coatings by direct chemical reaction is, for example, described in the following publication: Smith, W. D., "Boron Carbide Fibers from Carbon Fibers", Boron and Refractory Borides, Springer-Verlag Berlin, Heidelberg, N.Y., 1977. Chemical conversion reaction coatings produced by a direct chemical reaction with the surface of carbon fibers reduce the strength of the fibers by chemically altering the carbon fiber surface and are, therefore, of limited use.
In view of the above literature survey it is apparent that more satisfactory means of protecting carbon fibers and their interface with a matrix are necessary to prevent rapid degradation at high temperatures in oxidizing environments and thereby to exploit the full potential of carbon fiber-reinforced carbon-carbon, metal matrix and ceramic matrix composites.